Color-tunable transmission mode active phosphor based on III-nitride nanowire grown on transparent substrate

ABSTRACT

A system and method providing correlated color temperature-tunable (CCT-tunable) white light using a laser diode(s) in conjunction with a III-Nitride nanowires-based LED element grown on a semi-transparent substrate. The tunability spans across yellow, amber, and red wavelengths and can be implemented by current injection. The current-dependent broad wavelength tunability enables control of wide range of CCT values (intensity, peak wavelength, and spectral coverage). The broad coverage in the yellow-amber-red color regime mimics that of a passive yellow phosphor, while the injection of current into the LED element defines an active phosphor element. The semi-transparent active phosphor element allows direct transmission of light from a laser diode(s) for achieving extreme wide tunability of CCT.

BACKGROUND

Light emitting diode (LED) based solid state lighting has gainedtremendous popularity due to its advantages over traditional lightingtechnologies. For white light generation, multiple wavelengths arerequired to generate a broad spectrum of light which better approximatesthe black body curve. The conventional approach to white lightgeneration relies on down-converting blue light using YAG:Ce³⁺ phosphorto generate broadband yellow light. By mixing both the blue and yellowcomponent, white light has been produced with high theoretical efficacy.

Unfortunately, white light generation using yellow phosphor posesseveral limitations, such as quality reduction due to phosphor materialdegradation and the inability to optimize the intensity and colorrendering index. Furthermore, future demand for high speed opticalwireless communication is limited by the long carrier relaxation time ofthe YAG phosphor. Therefore, an alternative method for white lightgeneration capable of providing durable high quality lighting overextended periods of time and with color tunability has become anattractive goal.

Visible lighting and image projection based on solid state devices haverecently attracted considerable attention because of their smallfoot-print, long lifetime, stable light-output, low power consumptionand heat generation, and mercury-free manufacture. To achieve whitelight, most conventional techniques utilize blue LED to excite yellowphosphor or combining red, green and blue (RGB) LEDs to produce whitelight. As described above, phosphor based techniques suffer from limitedcontrollability of the yellow phosphor component in producing thedesired white light characteristics.

SUMMARY

Embodiments of the present disclosure describe a correlated colortemperature tunable (CCT-tunable) white light generation systemcomprising a laser diode or a combination of laser diodes in conjunctionwith a broadly tunable III-Nitride nanowires-based LED elementepitaxially grown on semi-transparent substrate. In one example, ayellow-amber-red III-Nitride nanowires-based LED grown on a transparentquartz substrate is used as the active phosphor and light from a bluelaser diode is used as an external light source which is then introducedthrough the transparent substrate for light mixing. In one embodiment,the external blue light is directed to a substrate underside and passesthrough the transparent substrate. By mixing light from the activephosphor with an external, secondary light passing through thesubstrate, it is possible to generate an extremely-wide tunable whitelight with a varying CCT from cool white (13782 K) to warm white (2812K) with a color rendering index (CRI) value as high as 74.5. In anotherexample, by further incorporating red, green, and blue light from laserdiodes as a combination of secondary wavelengths or light, a suitable ahigh quality white light can be generated at 7284 K with CRI as high as85.1, suitable for indoor lighting applications.

Additional embodiments describe an active phosphor based on an on-chiptunable yellow-amber-red (peak wavelengths of 590 nm to 650 nm)nanowires-based LED grown on a transparent quartz substrate. Thenanowires-based LED on quartz substrate emits at a center wavelength of590 nm. By mixing light from the yellow-amber-red LED with a secondaryred, green, and blue light from a laser diode source in transmissionconfiguration (herein described as transmission mode), a high qualitywhite light is produced having a CRI of 85.1. The active phosphor basedon nanowires-based LED on quartz substrate enables the realization ofhigh quality, extreme-tunable, durable, and compact white light source.

Prior light systems in U.S. Pat. Nos. 8,716,731 B2, and 8,647,531 B2teach mixing of different compounds to produce a phosphor elementemitting at different color which is limited because the color cannot betuned on-chip. US Patent Application Publication No. US2009/0153582 A1teaches the use of a combination of ultraviolet light laser sources withthe corresponding phosphor elements, in which the spectralcharacteristics of the phosphor elements cannot be actively tuned, thuslacking in the simplicity (single laser and single LED), faciletunability and durability that embodiments of the present disclosureprovide. U.S. Pat. No. 8,629,425 B2 teaches a method of monolithicintegration of two color LEDs, which is limited due to lack ofindependent tuning of the reported two color or wavelength componentsmonolithically integrated in a device, each embedded in the form ofquantum dots, resulting in the undesirable reabsorption of the bluelight produced.

In comparison, an embodiment of the present disclosure teachesindependent control of first and second wavelengths with the firstwavelength resulting from III-Nitride nanowires epitaxially grown on lowcost quartz substrate while exhibiting single crystalline quality andthus better control over the light emission line shape and CCT/CRI.

Prior nanowire based active phosphor systems were limited due to therequirement of reflective-geometry color mixing on opaque substrate,such as disclosed in U.S. Patent Application Ser. No. 62/375,748. Toaddress this problem, an embodiment of the present disclosure teaches atransmission mode active phosphor using a transparent substrate toenable a more flexible and direct approach in designing white lightsources. Transparent quartz substrates also have the added advantage ofscalability and direct integration with existing consumer devicescompared to conventional substrates currently used for light emitterfabrication.

The present disclosure presents a CCT-tunable white light generationsystem comprising a laser diode or a combination of laser diodes inconjunction with a broadly tunable III-Nitride nanowires-based LEDelement grown on transparent or semi-transparent substrate. The widewavelengths of the nanowires-based LED is broadly tunable and may coverthe ultraviolet regime, the visible regime, or the infrared regime. Thesecond or a combination of secondary light source(s) may include one orseveral laser diodes with narrow linewidth. The transparent substratemay include, but is not limited to, glass, quartz, fused silica, andsapphire.

Application of quality white light generated by devices and methods ofthe present disclosure include indoor lighting, automotive lighting,back lighting units, outdoor lighting, and monolithic-integratedmulticolor laser-nanowires LED chip for example.

BRIEF DESCRIPTION OF DRAWINGS

This written disclosure describes illustrative embodiments that arenon-limiting and non-exhaustive. In the drawings, which are notnecessarily drawn to scale, like numerals describe substantially similarcomponents throughout the several views. Like numerals having differentletter suffixes represent different instances of substantially similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

Reference is made to illustrative embodiments that are depicted in thefigures, in which:

FIG. 1 illustrates a conceptual rendering of color mixing of firstwavelength from the nanowires-based LED epitaxially grown on atransparent substrate and a second wavelength of the transmitted laserlight, thus resulting in white light.

FIG. 2 illustrates a conceptual color mixing scheme for white lightgeneration in accordance with one or more embodiment of the presentinvention.

FIGS. 3A-B illustrate spectral characteristics (electroluminescence, EL,intensity; full-width at half-maximum, FWHM; peak wavelength) as afunction of LED current injection.

FIGS. 4A-B illustrate CRI and the extreme wide tunability of CCT asfunctions of current injection of LED and blue laser diode.

FIG. 5 Illustrates the LED as the first tunable broadband wavelengthunder direct current injection illuminated by a second wavelength from ablue laser diode from the backside.

FIG. 6 is an illustration of color spectrum and CIE 1931 color spacediagram for mixing of a first broadband wavelength (yellow-amber-red)and second narrow wavelength (blue).

FIG. 7 is an illustration of color spectrum and CIE 1931 color spacediagram for mixing of a first broadband wavelength (yellow-amber-red)and secondary narrow wavelengths (red, green, and blue).

DETAILED DESCRIPTION

A III-Nitride nanowires based device has reduced defect density,improved light-extraction with a larger surface to volume ratio, andincreased internal quantum efficiency due to a reduced lattice-strain,thus considerably mitigating efficiency droop.

In one example, a III-Nitride nanowires on quartz sample was grown usinga Veeco GEN 930 plasma-assisted molecular beam epitaxy (PA-MBE) system.Organic contaminants were removed from 500 μm thick, 2 inch diametercommercial quartz substrate using acetone and IPA. After cleaning, ˜200nm Ti was sputtered on the backside to enhance the substrate's heatabsorption and uniformity. Functioning as a semi-transparent conductinglayer, 20 nm of Ti was deposited on the wafer's front side using ane-beam evaporator. The sample was then transferred to a MBE growthchamber where the temperature was increased to a growth temperature.Temperature was calibrated with a pyrometer. Before Ga deposition, thesample was positioned facing a nitrogen plasma source for 10 minutes atelevated temperature to partially convert the Ti metal layer into TiN.Silicon doped GaN NW was first nucleated at a lower temperature of 640°C. followed by growth at a higher temperature of 740° C. for crystalquality improvement. Nitrogen (N2) flow was maintained at 1 standardcubic centimeter per minute (sccm) with RF plasma power source set to350 W throughout the growth. An active region consisted of 5 pairs ofCraN quantum barrier and InGaN quantum disks. The NW structure wascapped with an Mg-doped p-GaN top as a p-contact layer.

Additional details of nanowires-based LED fabrication are disclosed inU.S. Patent Application Ser. No. 62/375,748, entitled “UltrabroadLinewidth Orange-Emitting Nanowires LED for High CRI Laser-Based WhiteLighting and Gigahertz Communications,” the disclosure of which isincorporated by reference herein in its entirety.

FIG. 1 illustrates a conceptual drawing of transmission mode colormixing using a transparent yellow-amber-red nanowires-based LED 10 as abroad tunable active phosphor and a blue light laser diode as a narrowlinewidth light source 12 to produce white light 14. Owing to thetransparency of the LED 10 substrate, it is possible to position thelaser light source 12 directly behind the LED 10 to provide anadditional degree of freedom in device and system design. In otherexamples of the present disclosure, the transparent substrate mayinclude glass, quartz, fused silica, and sapphire.

FIG. 2 illustrates a color mixing system for white light generation. Redlight is generated by red laser diode source 20, green light isgenerated by green light source 22, and blue light is generated by bluelight source 24. Light sources 20, 22, 24 can be laser diode sources. Awavelength combiner 26 combines the RGB light from sources 20, 22, 24.In one example, a Thorlabs 3-channel wavelength combiner was utilized.The resulting RGB beam is collimated using a collimating lens 28 andreflected with a 45 degree mirror 30 toward the back side ofyellow-amber-red nanowires-based LED on quartz 32. Diffuser 36 diffusesthe collimated laser beam to allow mixing of white light. The RGB beamis mixed with light from the active phosphor from yellow-amber-rednanowires-based LED on quartz 32 to generate a high quality white light34.

In one example, the III-Nitride nanowires-based LEDs on quartz with mesadimension of 500×500 μm² were fabricated using standard microfabricationtechnique. All electrical measurements were done at room temperature. Acolor mixing experiment utilized a III-Nitride nanowires-based LED onquartz as a current-dependent broad wavelength source, thus constitutingan active phosphor element, and red-green-blue (RGB) laser diodes as acombination of secondary wavelengths with narrow linewidth. The beamsfrom the RGB laser diodes were first combined together using a Thorlabs3-channel wavelength combiner, and then collimated using a collimatinglens. The collimated beam was then reflected using a 45° mirror onto thebackside of the III-Nitride nanowires-based LED on quartz, and passedthrough the LED's top side. The detector was positioned above theIII-Nitride nanowires-based LED on quartz to collect the wavelength ofresulting mixed color light. CIE and CCT were then calculated using GLoptics software based on CIE 1931 standard.

FIGS. 3A-B illustrate the broad tunability of spectral characteristicsincluding electroluminescence (EL intensity), full-width at half-maximum(FWHM), and peak wavelength as a function of LED current injection foran embodiment of the present disclosure. The wavelength coverage spansfrom 450 nm (blue) to 900 nm (near infrared). In FIG. 3A, LED currentinjection ranges from 4 mA to 26 mA. In FIG. 3B, LED current injectionranges from approximately 5 mA to 26 mA.

FIGS. 4 A-B illustrate the broad tunability of white light produced bymixing a narrow blue laser diode light with the active phosphor from aIII-Nitride nanowires-based LED on quartz. As shown, the LED currentinjection (FIG. 4A) and the laser diode current injection (FIG. 4B) werevaried to change correlated color temperature (CCT) and color renderingindex (CRI). The CCT and CRI were directly measured using a GL Spectis5.0 Touch spectrometer measurement system.

In another experimental setup, as shown in FIG. 5, a yellow-amber-redIII-Nitride based LED on quartz under direct current injection wasilluminated by a blue laser beam from the underside. The LED was placedon top of a transparent glass slide. The current injection for the LEDwas supplied by a Keithley 2400 source meter. A Thorlabs LP-450 SF15laser diode was used as the laser beam source.

White light obtained after mixing broadband yellow-amber-red light anddiffused blue laser light was evaluated according to the CIE 1931standard. FIG. 6 illustrates the white light spectra along with a CIE1931 color space diagram (inset). A CCT of 6769 K and CRI of 74.5 wereobserved. CRI provides a quantitative measure of the degree of a lightsource revealing the color of an object under consideration, whencompared to a Planckian light source having the same Kelvin temperature.In comparison, FIG. 7 illustrates the white light spectra and colorspace diagram for a RGB light source mixed with yellow light, where aCCT of 7284 K and CRI of 85.1 were observed.

The foregoing description of various preferred embodiments of thedisclosure have been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit thedisclosure to the precise embodiments, and obviously many modificationsand variations are possible in light of the above teaching. The exampleembodiments, as described above, were chosen and described in order tobest explain the principles of the disclosure and its practicalapplication to thereby enable others skilled in the art to best utilizethe disclosure in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the disclosure be defined by the claims appended hereto.Various examples have been described. These and other examples arewithin the scope of the following claims.

Other embodiments of the present disclosure are possible. Although thedescription above contains much specificity, these should not beconstrued as limiting the scope of the disclosure, but as merelyproviding illustrations of some of the presently preferred embodimentsof this disclosure. It is also contemplated that various combinations orsub-combinations of the specific features and aspects of the embodimentsmay be made and still fall within the scope of this disclosure. Itshould be understood that various features and aspects of the disclosedembodiments can be combined with or substituted for one another in orderto form various embodiments. Thus, it is intended that the scope of atleast some of the present disclosure should not be limited by theparticular disclosed embodiments described above.

Thus the scope of this disclosure should be determined by the appendedclaims and their legal equivalents. Therefore, it will be appreciatedthat the scope of the present disclosure fully encompasses otherembodiments which may become obvious to those skilled in the art, andthat the scope of the present disclosure is accordingly to be limited bynothing other than the appended claims, in which reference to an elementin the singular is not intended to mean “one and only one” unlessexplicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present disclosure, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims.

What is claimed is:
 1. A white light generator, comprising: an activephosphor element including a nanowires-based light emitting device (LEDemitting light across yellow-amber-red wavelengths, with nanowires ofsaid active phosphor element being epitaxially grown on one side of atransparent substrate; and a external light source emitting light at asecond wavelength outside of said yellow-amber-red wavelengths toward asecond side of the transparent substrate opposite the first side, withsaid light source being positioned away from the transparent substrate,and with light from the active phosphor element combining with lightfrom the external light source that passed through the transparentsubstrate to generate a wide-spectrum white light being emitting awayfrom the first side of the transparent substrate.
 2. The white lightgenerator of claim 1, wherein light from the active phosphor element istunable through current injection.
 3. The white light generator of claim2, wherein said light is tunable from 590 nm to 650 nm by adjustingcurrent injection while maintaining the narrow linewidth light sourceabove 120 nm.
 4. The white light generator of claim 1, wherein theexternal light source emitting light at the second wavelength is anarrow linewidth laser.
 5. The white light generator of claim 1, whereinon-chip tuning of the active phosphor element by current injectionenables tunability of correlated color temperature (CCT) valuesresulting from variability in light intensity, peak wavelength, andspectral coverage.
 6. The white light generator of claim 1, wherein acombination of fixing the light wavelength and tuning the secondwavelength external light source enables wide CCT-tunability ofgenerated white light.
 7. The white light generator of claim 1, whereinthe second wavelength is blue light at approximately 450 nm.
 8. Thewhite light generator of claim 1, wherein the external light sourceemits at wavelengths of red, at approximately 642 nm, green, atapproximately 520 nm, and blue, at approximately 450 nm.
 9. The whitelight generator of claim 5, wherein CCT values range from 5000 K togreater than 8000 K and color rendering index (CRI) is above 65 byadjusting current injected into the nanowires-based LED and fixing thesecond wavelength to approximately 450 nm.
 10. The white light generatorof claim 6, wherein CCT values range from 3000 K to 7000 K and colorrendering index is above 55 by adjusting current injected into theexternal light source and fixing current injected into thenanowires-based LED.
 11. The white light generator of claim 1, whereinthe substrate includes electrical contacts for current injection whiletransmission of the second wavelength.
 12. The white light generator ofclaim 1, wherein the white light generator is housed within a lightingenclosure or vehicle light system.
 13. A white light generation systemcomprising: a transparent substrate having a first surface and a secondsurface; a plurality of nanowires-based LEDs grown on a first surface ofthe transparent substrate and emitting light away from the firstsurface; and at least one external light source positioned away from thetransparent substrate and with light from said at least one externallight being directed toward the second surface and through thetransparent substrate and exiting the first surface and then combiningwith light from the plurality of nanowires-based LEDs to produce a whitelight being emitted away from the first surface.
 14. The white lightgenerator of claim 13, wherein the light from the plurality ofnanowires-based LEDs is tunable through current injection.
 15. The whitelight generator of claim 13, wherein the at least one external lightsource is a laser.
 16. The white light generator of claim 13, whereinCCT values range from 3000 K to 7000 K and color rendering index isabove 55 by adjusting current injected into the at least one externallight source and fixing current injected into the plurality ofnanowires-based LEDs.
 17. A method of white light generation comprising:emitting light from a plurality of nanowires-based light emittingdevices LEDs epitaxially grown on a first side of a transparentsubstrate; directing light emitted from a secondary light source towarda second side of the transparent substrate opposite the first side, withat least some light from the secondary light source passing through thetransparent substrate; and mixing light from the secondary light sourcethat entered the transparent substrate at the second side and thenpassed through the transparent substrate with light from the pluralityof nanowires-based LEDs to produce a white light combination beingemitted away from the first surface.
 18. The method of claim 17, whereinsaid directing light includes directing light through a collimating lensprior to said light entering the second side of the transparentsubstrate.
 19. The method of claim 17, wherein said directing lightincludes reflecting light with a mirror positioned between thetransparent substrate and the secondary light source.
 20. The method ofclaim 17 further comprising, tuning white light characteristics byadjusting light emitted by the plurality of nanowires-based LEDs orsecondary light source or both via current injection.